Once the bug found by thyseus is fixed, this extension works fine. However, it would have been nice to wrap the latest version of the tipsy plugin (1.0.0a) instead of version 0.1.7.
I copy&paste your code sample and found that the 'sw' gravity is not working. It seems that only the latest version support composite gravity values like 'nw' for 'north-west' ... version 0.1.7 does not.